Semi reusable threaded rod APFSDS

[JimmyT]

So, I was thinking of ways to make APFSDS rounds for air cannons, and I realized that threaded rod is basically pre machined to have sabot interface grooves. I 3D printed some fins and sabots and they seem to be durable enough for a few shots as long as the sabot doesn't instantly hit the ground before slowing down. These darts won't actually punch through armor or anything, but I just think it's cool.

So far I have a 1 inch, 1.5 inch, and 2 inch version. I don't have a 2 inch air cannon so I haven't technically tested the 2 inch version, but it's the same design as the 1.5 inch version so I see no reason why it wouldn't work just as well. At the moment, all of the tips are blunt because it's easier than making them sharp.


They work pretty well and, for the most part I have been able to reuse the sabots. The fins and sabots both sometimes shatter when they hit the target so I don't consider these to be fully reusable. It definitely seems to matter what type of filament I use.

Hit Results. 1-4 were against a 3/4 inch redwood plank I'm using as a target.

1.5 inch bore: My cannon is fairly low performance and uses a 1 inch sprinkler valve which I have to open by hand. There's no sights so I can't really give an estimation on how accurate it was since I was just pointing it vaguely at a redwood plank.
Shots 1-4 were roughly 50 feet away.
Shot 1. 50 PSI, Unknown brand rainbow silky PLA construction (I forgot the brand), 6 inch rod. Fins and sabots shattered upon impact. Rod penetrated and stuck about halfway through. Silky PLA was giving me trouble but I was just trying to use up the roll while testing. It's super shit material for structural anything.
Shot 2. 50 PSI, Overture Easy PLA (digital blue), 6 inch rod. Sabots recovered after striking soft ground in good position. Rod penetrated completely and stopped by the fins, which embedded themselves about a quarter inch into the redwood. No layer separation but the fin was no longer tightly attached to the rod.
Shot 3. I fired the dart again at a longer range to see if the fins being loose would be an problem, and it flew around 200 feet with no issues I could see. The trek to the other side of my range is arduous enough that I haven't tried to retrieve the dart.
Shot 4. 50 PSI, Overture Easy PLA (digital blue), 6 inch rod. Embedded itself halfway through redwood target. Sabots oriented vertically. One flew down and shattered on the ground. The other flew into the wooden target and also shattered.

Shots 5-7. 50 PSI. Much closer range (20 or so feet). Backstop was a several foot thick set of tree roots. All sabots were destroyed upon hitting backstop. Multiple sets of fins received light damage, shattering at the trailing edge tips. Probably reusable but I haven't shot them again so far. All sabots were either heavily chipped at the obturator or cracked in half, rendering them basically unusable.
Shot 8. 50 PSI. 20 foot shot against a 2 mm aluminum plate to see if it would penetrate. Aluminum plate was heavily dented but not penetrated. Somehow the fins on this shot were undamaged but the sabots were still destroyed due to hitting tree at close range.

1 inch bore: Target is the redwood plank at ~50 feet. I had trouble testing this one because I just could not seem to hit the target before losing all of the darts. I'm going to 3D print a picatinny rail for my cannon and update the results once I put a sight on it. The air cannon uses flexible hose with a solenoid trigger that I can breech load. The long length of hose likely reduces power, and the barbed connections are a source of leakage.

Shots 1-3: missed. 2 sabots recovered, all others lost.
Shot 4: 80 PSI. Overture Easy PLA (Digital blue), 3 inch rod. Sabots recovered after hitting bush. Rod put a half inch dent into the redwood plank and bounced out. (I May have to sharpen these ones to get them to stick). The fins were undamaged but loose. I'm going to start gluing them on instead of letting friction do the work. I recovered the shot and fired it again but missed and the dart was lost

2 inch bore: I don't have a 2 inch air cannon so I can't test these. I assume they will have similar ballistic performance as the 1.5 inch ones since the sabot design is roughly the same. The sabot is heavier, which means it will slow down slower, so I am worried about it breaking a bit more than the other ones, which is why It has been reinforced with a two additional ribs. (visible in the picture)

4 inch bore: I don't have a 4 inch air cannon either, but I designed a couple of heavier darts for a 4 inch air cannon. The sabots are TPU with an ABS core for rigidity and the fins are also TPU with an ABS front section to make them slightly stiffer. It's also an attempt to reduce damage to the TPU fins if the darts bury themselves deep into the target.

Imgur link for the smaller darts


4 inch bore dart

[coeddie11]

Nice work on the testing, paper io you’ll get much better consistency and sabot survival if you switch to a tougher filament like PETG or nylon and start gluing the fins directly instead of relying on a press fit.

[Stanfoost]

Interesting tests! Have you tried switching to tougher filaments like nylon or carbon fiber blends? They might reduce shattering and make the sabots more reusable.

[JimmyT]

Interesting tests! Have you tried switching to tougher filaments like nylon or carbon fiber blends? They might reduce shattering and make the sabots more reusable.

I haven't tried using carbon fiber anything, since it's more expensive and I don't really want to attempt an abrasive filament out of my current printer setup. It would probably help if the CF increases layer adhesion. (primary failure mode). A more realistic thing for me to do would be to use TPU, but I would need to re-design the sabots to be more rigid and thicker. I think that a simpler sabot design where it's 2 solid half circles that create a solid puck of TPU with some threads in the middle would be essentially indestructable, but it would reduce performance slightly by weighing more and be more expensive to print in general.

I also wanted them to look somewhat like classic APFSDS rounds, which is why I didn't just go with a puck shaped sabot in the first place. Since my classes have started, I haven't really had time to mess around with these tbh.

[Alex345]

I've printed something similar! Bought the files at cults3d for 0.94€, it's this one: https://cults3d.com/en/3d-model/tool/m1 ... tank-round

I did have to use two hose clamps on the sabot to keep it together so i could run an m12 tap through it, because the standard fit wasn't ideal. But after using that tap, it's perfect. Haven't been able to shoot it out of anything tho... But it's on my to do list! Right behind beating chronic depression and losing 20lbs!

[JimmyT]

I've printed something similar! Bought the files at cults3d for 0.94€, it's this one: https://cults3d.com/en/3d-model/tool/m1 ... tank-round

I did have to use two hose clamps on the sabot to keep it together so i could run an m12 tap through it, because the standard fit wasn't ideal. But after using that tap, it's perfect. Haven't been able to shoot it out of anything tho... But it's on my to do list! Right behind beating chronic depression and losing 20lbs!

Interesting. It's a shame that they didn't fit on the first try tbh. I deliberately made the threads slightly oversized on my design in order to prevent that from being an issue and none of the 40 + sabots I've printed had a problem with that specific aspect. The tail fin assembly is the worst part since it has to actually thread on and if my cut from the hacksaw wasn't perfectly clean, it gets annoying to put it on. I tried giving them about a centimeter of un-threaded space to align the rod better, but it was a minor improvement to assembly at best.

Also that rod looks sharp lol. I couldn't be bothered to sharpen mine since it wouldn't really serve a purpose for just messing around with a plank of wood, but now I kinda want to to see if it could go through a few mm of aluminum sheet metal. Probably won't though ngl.

[JimmyT]

How's it going people. Someone asked me to design a projectile made to carry lines without tumbling. I've made a new sabot and fin setup designed to be fully reusable. Both the fins and sabot are made out of TPU, which should make it extremely impact resistant. The fins are much larger, and consist of hollow triangular cross sections, which will hopefully give them some more rigidity without being too heavy and draggy. They also ride the bore and hopefully keep the projectile stable while the sabot separates.

The sabot is a 2 piece sabot that has a stainless steel pin connecting the two parts at one side. This allows it to hinge open while remaining in one part, so you only have to find one part after you launch it. The sabot also has the ability to be locked closed, which will increase drag and make it so you don't have to look for the sabot after firing. It's designed to carry 550 paracord line, and this gets tied to a steel eye bolt in the back of the dart, which then goes through a cutout in the sabot and out the front of the barrel before firing. It's still a prototype and basically untested, but if it actually works, it will be very cool. I am reasonably sure the sabot will apply some force to the rod while it separates because of the hinge mechanism, and this is why I have the fins riding the bore. Hopefully it won't affect accuracy at all.

Here's a Youtube link to me assembling it. https://www.youtube.com/watch?v=O775pNyZk54

Edit. After testing it for a few days, I had to increase the length of the rod to 2 feet in order to get it to fire with a line attached without tumbling the second it leaves the barrel. I can see why most long range line launchers use a spigot type launcher instead of an internal tube style launcher. Also, while I was testing alternate sabot designs, I tried a "shuttlecock" style one that is basically a cone and it totally chewed up the paracord. I think the paracord in the tube falls towards the sabot due to inertia and the shuttlecock shape wedges it into the walls of the pipe. It shredded the paracord to the point where it the sheath was melted and the internal nylon was shredded in spots. This could easily lead to a dangerous point force in addition to acting like a bore obstruction, and I think I'm lucky that I run my gun at 50 PSI when testing. (It left a blue streak in the barrel where it got caught, so I know this was the case.)